City Square

City Square
Brookfield Multiplex
Techniques Used: 
Concrete Injected CFA Piles

Project Brief: The City Square project at 125 St. Georges Terrace involved the construction of a 46 level plus 4 level basement office building, with the BHP Billiton being the anchor tenant. Once the 235m building is opened in 2012, it will become the second tallest building in Perth.

Geological Conditions: The site is located in Perth CBD, between St Georges Terrace and Mounts Bay Road.  The site consisted generally of interbedded sands and clays overlying the siltstone of the Kings Park Formation. The foundation piles were designed to socket into the underlying siltstone located approximately 18m from the piling platform level.

Scope of Work: The works comprised the design and construction of just over 450 No. 750 mm diameter CFA piles. All piles were installed with full computer instrumentation displayed in real time to the operator to ensure pile quality and structural integrity.  All computer pile records were also reviewed in weekly meetings with the clients structural and geological consultants due to the nature of the project.

A major feature of the building is its four mega columns, located in each corner of the tower, with up to 13 No. piles supporting each of these columns. The remaining piles were located in groups ranging from two to eleven piles.

The building has a relatively small footprint, resulting in a significant number of piles located in close proximity. This created a constructability issues given the size of the piling rigs required to install the piles. This issue was resolved with all pile reinforcements being installed with couplers at cut-off levels. The reinforcement projection bars were installed (screwed in) using the couplers after the construction of all piles and prior to the construction of pile caps and rafts.

To add further complexity to the foundation designs, 64 No. 1.4m diameter piles, socket into the siltstone, had been constructed across the site 10 years ago, to support an aborted high-rise development. This required the individual design of a large number of pile caps to support heavily loaded columns, and trying to avoid the influence of these previously constructed piles.

Conclusion: A cost effective and innovative solution for the foundation for the City Square project was brought about by co-operation between consulting structural and geotechnical engineers and piling contract. Ground conditions were complicated on site by the need to design the new foundations around the aborted foundations of an earlier development.

The adoption of CFA foundation piles allowed significant cost and program savings to the client, allowing the project to make up lost time.